1. Field of the Invention
The invention concerns to a procedure for the photometric determination of the quality of gas, particularly of burnable gases, and also devices for the photometric determination of the quality of gas, particularly of burnable gases.
2. The Prior Art
For the registration of the quality of gas for example in distribution networks for natural gas or the same already for a long time devices for the registration of the quality of the guided through gases are used, so-called devices for measuring the condition of gas. Natural gas shows because of being a natural product according to its origin and by mixture respective fluctuations in respect of its composition, whereas the composition for example of natural gas coming from the different hydrocarbons determines essentially the calorific value and there from extrapolated quantities. Therefore it is of great importance for the account of the guided-through amount of gas in a gas supply network and therewith the respective amount of energy, to determine exact the respective condition at the feeding point into the natural gas network and at the deliverance points of the customers and therewith to deduct a definite transported or supplied amount of energy. In doing so for the customer of the gas an invoice can always be stated according of the actual supplied amount of energy regarding to different conditions of the gas and a correspondingly varying amount of energy. Vice versa the detection of the condition of the gas offers the guarantee for the customer, to checkably obtain a desired quality and therewith a required amount of energy.
The registration of the quality of gas obtains additional relevance, since with the drop of the guiding-through monopoly the suppliers of natural gas guide through the same network for delivering gases of quite different provenance and therefore also different composition. Only an as far as possible easy and cost-effective detection of the condition of the gas by means of corresponding disposed cost-effective measuring devices and methods of measurement allows therefore a controllable and accurate accounting.
For the registration of the quality of gas as relevant quantities the standard volumetric gross calorific value Hv.n, the standard density ρn and the compressibility coefficient K have to be determined as correct as possible and also regarding the different gas qualities.
In practice for the energetic settlement of accounts first of all by means of flow measuring devices the transported volume Vb of the gas at working conditions (pressure ρb, temperature Tb) is measured. With knowledge of the condition of the gas the compressibility coefficient K can be determined, with which the volume of the gas Vn at standard conditions (pressure ρn, temperature Tn) is calculated.   Vn  =                    ρ        b                    ρ        n              ⁢                   ⁢                  T        n                    T        b              ⁢                            1                                      K                      ⁢                   ⁢          V      b      
By means of multiplication of this standard volume with the volumetric gross calorific value Hv,n at standard conditions the transported amount of energy Q can be obtained:O=VnHv,n
Alternative the volume at working conditions vb can directly be multiplied with the volumetric gross calorific Hv,b at working conditions (Energymeter).
Another important quantity for applications with natural gas is the thermal output of gas burners; this varies in accordance to the gas quality and is characterised by means of the so-called Wobbe index Wv: gases with the same Wobbe index Wv deliver the same thermal output at a burner nozzle. For calculating the Wobbe index Wv the standard density ρn of the gas is required, from which the relative density according to air is determined (dv=pgas/pair)   Wv  =            H      v                      d        v            
Therefore the determination of the gross calorific value Hv,n at standard conditions has central relevance for the practical determination of the quality of gas for example for accounting purposes.
Until now different devices for the registration of the gas quality are used. So-called direct and so-called indirect procedures are known. By using direct procedures the quantities to be determine dare measured separately and therefore the gas is transformed to standard conditions, by which partially expensive treatments of the gas are required.
At the most easiest way the gas condition can be determined by means of so-called calorimeters, in which by means of an open flame a gas probe is burnt and out the arising and to a cooling medium submitted heat quantity and the thereupon detectable temperature rise of a cooling medium the calorific value of the burnt gases can be determined. Such devices will need a complicated mechanic for the adjustment of a certain quantitive proportion of gas, combustion air and for example cooling air as cooling medium and are therefore expensive and error prone, especially relating to the open burning enhanced security requirements for the devices are necessary. Also the maintenance and calibration has to be carried out by qualified personnel, beyond that the calorimeter must be used in conditioned rooms. Therefore, the purchase and operating costs of such test assemblies are very high.
Using calorimetry by means of catalytic burning (for example with pellistors) the probe gas is mixed with air and burnt at the 400 to 500° C. hot helixes of a catalyst. The temperature rise of the catalyst is about proportional to calorific value. Because this procedure is based on a sensitive surface effect, it is subject strongish drifts and necessitates frequently calibration with search gas. The catalytic calorimeter are most favorable of all here presented procedures, however they are better suited for control than for accounting) because of their accuracy.
The direct measurement of the density Pb at working conditions is done in one way with hydrostatic balances, very expensive precision devices, with which the buoyancy of a ball filled with nitrogen is measured in accordance to the density of the surrounding medium, here of the probe gas. With another procedure a thin-walled metal cylinder, which is positioned by a current linkage of the probe gas, is set in oscillation. The density of the surrounding gas determines the resonant frequency of the cylinder, which is captured as sensitive measured quantity. Both procedures are very expensive for the determination of the standard density, because you they require an adjustment to the standard conditions.
The compressibility coefficient K can in this way not be measured directly, instead can be calculated by means of different numerical standard-arithmetic techniques out of the directly measurable gas quantities. One of these procedures, the so-called GERG88-procedure (DVGW-worksheet 486) needs thereby the input quantities listed in table 1. The amount of substance of CO2 is determined according to the today's state of the art by a non dispersive infrared-spectroscopical procedure (NDIR), whereby the gas must be brought into a defined condition near or at standard conditions. The amount of substance of H2 is practically of significance only when working with coke oven gases and can be practically left unattended in the typical natural gases today distributed in Europe. The compressibility coefficient K can be determined to 10−3 with the help of the GERG88-equation in case of sufficient accuracy of the input quantities.
TABLE 1input quantities of the GERG88-procedurePbpressure at working conditionsTbtemperature at working conditionsρndensity at standard conditionsHv, nvolumetric gross calorific at standardconditionsXCO2amount of substance of CO2xh2amount of substance of H2
The other procedure for determination of the behaviour of real gases is done according to the AGA8-92DC-equation equation (ISO 12213-2:1997 (E)). This process requires as input quantities the amount of substance of 21 leading gas components (table 2) and gains just as an accuracy of 10−3.
TABLE 2input quantities of the AGA8-92DC-equationmethaneCO2ethaneN2propaneH2SisobutaneHen-butaneH2OisopentaneO2n-pentaneArn-hexaneH2n-heptaneCOn-octanepressuren-nonanetemperaturen-decylhydride
The state of the technology includes besides the direct measurement techniques also the indirect measurement of the gas quality by means of gas chromatography. Thereby a defined volume of the probe gas is brought into a defined condition and is carried by a carrier gas, typically helium, through a system of gaschrornatographic separation columns. On account of their different times of retention the individual gas components reach the downstream sensor, generally this is a detector for caloric conductibility, at the end of the separation column separated in time. The peak area of the sensor signal can therefore be interpreted as amount of substance, whereas the evaluation must be carried out in comparison with a reference gas, that must have just about a similar composition like the probe gas. The drawback of the gas chromatography has its reason in the expensive sample preparation and installation of the whole system, and in the expensive maintenance and operation by well-trained personnel. From the amounts of substances of the individual gas components, as the gas chromatography delivers, all relevant gas quantities can be calculated. For the implementation of such indirect measurements in the way of chromatography automatically working process chromatographs with detectors for caloric conductibility are deployed. These devices measure normally eleven components of the natural as (N2, CO2, CH4, C2H6, C3H8, C4H10, C5H12, C6+ and so on). As carrier gas helium will be used, whereby its light volatility in practice often leads to prematurely emptying of the bottle for the carrier gas and therefore leads to short maintenance cycles for such a device for measuring the quality of a gas. As calibrating gas a gas is chosen, that is similar to the natural gas to measure. Such chromatographic systems carry out measurement cycles without interruption, in order for capturing changes in the quality of the gas immediately. These leads to a high consumption of carrier gas and calibrating gas and has moreover the effect, that maintenances of the device have to be performed in relatively short intervals.
It is also known, to determine the composition of a gas with conventional infra-red-gas-analyzers. Such analysers working in the middle infra-red or near infra-red area offer however not the requested requirements to high precision and in particular to stability for a determination of the caloric value under the measurement conditions which are required here. Also always a parallel reference measurement is to be carried out beside the intrinsic measurement of the probe gas, for the sake of compensating the at least essential influences of failures. As a measuring result the known infra-red-gas analysers deliver superimposed frequency spectrums, that make very difficult a conclusion to individual components of a inspected gases, if not even forbid such a conclusion.
In the literature also has been described a infrared spectroscopical procedure for gas analysis (“Optical BTU sensor Development”, Gas Research Institute GRI-93/0083), that determines by means of so-called multivariate analysis (MVA) of the near infrared spectrum of gases the volumetric concentration of the amount of substances of the containing carbon components of the gas and therewith of the volumetric calorific value under operating conditions. This procedure delivers however not the calorific value Hv,n under standard conditions and not the standard reference density ρn and the amount of substance Of CO2, so that it is not qualified for the determination of the compressibility coefficient K and therefore not for the complete determination of the quality of a gas. By the determination of the calorific value by means of known photometric methods moreover minor claim to the necessary instrumental equipment for the realisation of the procedures are required, whereby a benefit in the speed of the coverage of the absorption spectrums of the natural gas in near or middle infrared spectral area is obtained. The entire absorption spectrum of the natural gas is therefore put additive together from of the sum of single spectrums of components represented in gas and therefore can be measured and be analysed with the aid of more appropriate methods of spectral analysis. Doing this the ascertained quota of extinction of a component in the entire spectrum of the natural gas is in effect equivalent to the part of the concentration of this component within search gas (so-called Beer-Lambert-law). With the knowledge of the calorific value of this respective component the calorific value of the entire mixture of gases can then be calculated as a summation value. Problematically in this procedure of spectral analysis is however the intense overlap by absorption bands of different components, which frequently lead to inaccurate results and beyond that to a high calculation effort.
One more infrared spectroscopical procedure concerning to the DE 198 38 301 Al will act as a direct spectral evaluation (DSA) with a spectral function, with which the spectrum of the gas is folded. The procedure allows the specification of the volumetric calorific value Hv,n under operating conditions direct from the spectrum. Herewith it is utilized, that while burning the gas the respectively caused heat of the reaction is based on the combustion of C-H-bindings and a thereby caused heat quantity depends to the present binding energy. This is thereby exploited, that the oscillations of the C-H-bindings, which show an equal to each other, certain binding energy and produce the same heat quantity during a combustion, interact with an associated wavelength of an electromagnetic radiation. Hereby it is possible by means of a wavelengths-resolved measuring and a wavelengths-dependent valence of the grade of interaction of these oscillations to calculate the calorific value Hv,b of the gas, without the requirement of an identification of individual gas components. In this document it is additionally proposed a device for the realisation of such a procedure, which is tuned for the specific requirements of the measuring method and enables a weighted summing up of the grades of interaction. With this procedure indeed the calorific value Hv,b of a mixture of gases under operating conditions can be determined, the other quantities relating to the determination of gas quality can itself not be determined with measuring techniques.
In the publication of G. Buonanno et al. “The influence of reference conditon correction on natural gas flow Measurement” in the British magazine “Measurement” vol. 23, No. 2 dated 1st Mar. 1998 (published of the Institute of Measurement and Control, London) on the pages 79-81 is indicated a procedure for determination on the conversion of gas volumes, in which the compressibility factor Z is determined iterational according to the procedure SGERG-88 und afterwards the parameters {umlaut over (ε)} and χ are calculated. These parameters are mathematically directly connected with the input quantities pressure p, temperature T an the amounts of substances.
Further in the U.S. Pat. No. 4,958,076 and the U.S. Pat. No. 5,822,058 photometric devices with filtering units are known.
After having today's state of the art there is no, especially no uniform procedure, that determines under operating conditions the substantial quantities volumetric standard-calorific value Hv,n, standard-density ρn and compressibility factor K.